They're doing it because unions pay people while they're on strike, their war chests aren't big enough to maintain a full strike for long, and striking members

They're doing it because unions pay people while they're on strike, their war chests aren't big enough to maintain a full strike for long, and striking members might not be eligible for unemployment (this varies by state). Picketing all three automakers at once might have been a mistake because it means any strike will ultimately be shorter-lived.